Filamentary materials include fibers in single and multiple strands, flat bands, or tubing produced in long lengths and conveniently wound on spools. The various filamentary materials may be either natural or synthetic fibers, glass or metal. Such materials are commonly utilized as reinforcements for plastic or elastomeric compounds or may themselves be fabricated into integral items as in the textile industry or the tire industry. Regardless of the application, it is customary to withdraw the filamentary material from the spool at or near the location it is being used. To facilitate such removal, the spool is customarily mounted on a spindle or let-off device which permits the spool to rotate as the filament is withdrawn.
A main function of a tension control device is to provide a uniform tension of the filament as it is withdrawn from the spool. This requirement applies also when the weight and diameter of the filament wound upon the spool decreases as the filament is consumed, and/or if the speed of withdrawal is changed. Furthermore, it is necessary that in a system employing multiple tension control devices that the withdrawal tension be substantially uniform among all devices. Another function of the device is to apply additional tension (or braking) when withdrawal is stopped, thereby minimizing unraveling of the filament on the spool because of the momentum of spool and its content. Such braking, in the stopped condition, also may serve to keep the spindle rotationally stable during loading of spools thereon.
Numerous braking devices have been developed for use with creels. Many of these provide for the filament to be payed out under tension greater than what is required for payout from the spool. As the tension decreases, with slack in the filament, the braking force is applied to slow the rotation of the spool. Further, the amount of tension to be maintained in the filament must be variable in order to accommodate operations with different filaments under various conditions. In the past, such creels having variable tension control have often required multiple individual adjustments and have not been desirably compact. Some designs have even required tension adjustments during payout of the filament, as the spool is emptied. In other instances, creels have exhibited undesirable hunting or loping in the form of periodic variations about a desired tension, particularly in high-tension applications.
One of the more commercially successful tension control devices used in the tire industry is in accordance with Applicant's U.S. Pat. No. 3,899,143. That device has a support structure which carries a spool support and a separately mounted rotatable pivot shaft. A first lever arm fixed on the pivot shaft carries a guide for tensioning the filamentary material as it is withdrawn from a spool mounted on the spool support and a brake which selectively engages the spool support. A second lever arm fixed on the pivot shaft is operatively connected with an air cylinder which effects a biasing that is transmitted to the first lever arm via the pivot shaft.
Tension control devices according to U.S. Pat. No. 3,899,143 have demonstrated exemplary operating characteristics under a variety of conditions and with a variety of filaments. However, there are several situations in which these tension control devices are not well suited. It has been found that the control arm and guide roller are vulnerable to damage from over-tension possibly caused by entanglement of the spooled material. In instances where the filamentary material is a heavy gauge wire, the guide roller imparts a “cast” or distortion to the shape of the wire. This may lead to a less than satisfactory end product or the need to provide additional manufacturing equipment to straighten the wire. To the present time, there has been no comprehensive device for adequately dispensing heavy filamentary material from a spool. Yet a third problem is that the control arm and roller inhibits closely mounting the multiple tension controllers on the creel assembly.
One way to overcome the foregoing problems associated with the prior art is to provide a tension control device in which the spool is carried by a pivotably mounted spindle assembly that is moveable with a pivotably mounted braking assembly as seen in U.S. Pat. No. 6,098,910. By utilizing a fixed cam that engages the braking assembly, the rotation of the spindle is inhibited whenever a predetermined tension force is absent from the filamentary material. The braking assembly is provided with a slidable block with cam bearings that are spring-biased against a curvilinear cam surface provided by the cam. This provides a gradual yet firm application or removal of a braking force depending upon the amount of tension applied to the filamentary material. The braking force, applied through the cam, adjusts in response to the varying tension of the material as it unwinds from the spool. An increasing tension accordingly acts on the pivotably mounted spindle assembly causing the braking force to be relieved by an increasing amount, thereby tending to keep the filament in constant tension; conversely, a decreasing tension causes a greater braking force to be applied, with full braking (within the limits of the device) at zero tension. Although an improvement in the art, the aforementioned tension control devices with a pivotably mounted spindle utilize a pendulum motion to provide displacement of the spindle and spool. However, such pendulum motion imparts the effect of gravity on the operating tension because the force from gravity varies according to the angular displacement. As a result, the force from gravity can be several times the desired tension output of the device.
It is also known in the art to use a magnetic eddy current brake to provide back tension of a spool from which filamentary material is withdrawn. In one known device, an eddy current disk rotates with the spool and a control arm is pivotally mounted near the spool. The filamentary material passes over a guide roller mounted to one end of the control arm. An opposite end of the control arm carries the magnetic material. The tension in the filamentary material is defined over the force to pivot or move the control arm. The amount of this force can be adjusted by a pressurized diaphragm cylinder. If the filament's tension exceeds the control arm force, then the magnetic brake material moves away from the eddy current disk and the braking force on the spool is reduced. If the filament's tension is less than the control arm force and that of the diaphragm, then the magnetic brake material moves toward the eddy current disk and the braking force on the spool is increased. However, the use of a control arm has the problems previously mentioned of imparting distortion to the filamentary material, damaging the guide roller from over-tension and preventing such devices from being closely mounted to one another on the creel assembly.
In view of the shortcomings of the aforementioned devices, there remains a need in the art for a tension control device that minimizes the force from gravity while still providing the benefits of a device that does not employ a control arm and guide roller.